U.S. Pat. No. 4,021,167, issued for an invention of Niimi et al., discloses an apparatus for manufacturing hollow spherical particles. The apparatus provides a large number of individual linear water jets arranged in a ring and converging at a single point so that a molten (ferrous) system containing graphite may be passed through this ring of water jets and the converging point. The carbon in the molten metal particles reacts with dissociated water molecules to form carbon monoxide and dioxide in the particles. Those gases, plus the freed hydrogen, oxygen, and sulfur dioxide gas, are said to form hollows in the particles. The particles range in size from 0.1 mm to 18 mm.
U.S. Pat. No. 4,565,571, issued for an invention of Abbaschiaw, discloses a method for producing hollow metallic spheres. Porous articles of sufficient strength are formed of a particulate material containing at least one electrically conductive metal. The porous article is subjected to an electromagnetic field which has a frequency sufficient to induce in the article an eddy current of such intensity to produce heat sufficient to melt the electrically conductive material. Heating of the molten article is continued for a time sufficient to expand any gas contained in the pores to a volume such that all of the entrapped pores combine to produce a hollow molten metal sphere, and then the sphere is cooled to solidify the molten metal.
U.S. Pat. No. 4,548,767 issued for an invention of Hendricks, discloses a method for producing small hollow spheres, the microspheres being made of glass, metal or plastic. In accordance with the invention, the sphere material is mixed with or contains as part of the composition a blowing agent which decomposes at high temperature. A droplet generator forms uniform size drops which then fall into an oven where water is removed leaving a solid particle. The solid particle then falls into a higher temperature zone of the oven where it is melted and the blowing agent decomposes. The gas from the decomposition blows the molten bubble into a microsphere of diameter ranging from 20 to 103 micrometers.
U.S. Pat. No. 4,162,914, issued for an invention of Cremer, et al., discloses a process for making hollow metal microballoons that can be filled with deuterium and tritium and used as laser and electronic beam targets for a fusion reaction. The process involves the formulation of clean, unoxidized metallic powders followed by inflation of the particles in a plasma arc. Water is introduced into the plasma so that the water disassociates partially into nascent hydrogen and nascent oxygen. The hydrogen is absorbed by the molten metallic particles. Subsequently atomic hydrogen dissolves and/or becomes molecular hydrogen and inflates the particles as they cool. The process produces microdiameter spheres in the range of 50-1000 micrometers.